1. Field of the Invention
The present invention relates to a technique for positioning a printing plate in an image recorder for recording a printing image on the printing plate.
2. Description of the Background Art
An image recorder known as a CTP (Computer to Plate) or a platesetter records an image for each color on a printing plate for use in printing. After the image recording, the printing plate is attached to a plate cylinder of a printing machine. A plurality of printing plates are used to print a plurality of images of respective colors one upon another on a single printing sheet, thereby forming a multi-color image on the printing sheet.
The operation of attaching a printing plate to the plate cylinder of the printing machine is performed with reference to punched holes for registration or positioning which are previously formed in predetermined locations in the printing plate.
All printing plates must have the same positional relationship between the punched holes for registration and recorded images on the printing plates, or a misregistration would occur between the images transferred onto the printing sheet. The conventional operation will be described with reference to FIGS. 22A, 22B, 22C, 23A, 23B and 23C.
The conventional operation is performed using an image recorder 110, a punching apparatus 120, and a printing apparatus 130.
Referring to FIG. 22A, the image recorder 110 generally comprises a recording drum 111 for mounting a printing plate P thereon; a recording head 112 for directing an optical beam modulated in accordance with an image signal onto the printing plate P; a motor 113 for rotating the recording drum 111; and a sub-scanning motor 114 and a feed screw 115 for moving the recording head 112 in the axial direction of the recording drum 111. A first pin 116 and a second pin 117 for positioning the printing plate P are mounted upright on the surface of the recording drum 111.
The printing plate P is positioned on the outer peripheral surface of the recording drum 111 by bringing an edge Pe of the printing plate P into contact with the first and second pins 116 and 117, and is secured on the outer peripheral surface of the recording drum 111 by a vacuum suction means or other securing means.
Thereafter, the recording head 112 records an image IM on the printing plate P. It is assumed that the recorded image IM is an image IM (C) corresponding to the cyan color in this case.
After the completion of the image recording in the image recorder 110, the printing plate P is removed from the recording drum 111, and is transported to the punching apparatus 120. As schematically illustrated in FIG. 22B, the punching apparatus 120 comprises a punch table 121 for securing the printing plate P thereon; a first punching registration pin 126 and a second punching registration pin 127 for positioning the printing plate P on the punch table 121; and a punch unit (not shown) for punching the printing plate P.
The printing plate P is positioned on the punch table 121 by bringing the edge Pe thereof into contact with the first and second punching registration pins 126 and 127. Thereafter, the punch unit punches the printing plate P to form punched holes Pi and Pj.
Next, the printing plate P is transported to the printing apparatus 130. As schematically illustrated in FIG. 22C, the printing apparatus 130 comprises a plate cylinder 131 for placing the printing plate P therearound; and a first printing registration pin 136 and a second printing registration pin 137 for positioning the printing plate P on the plate cylinder 131. Although not shown, the printing apparatus 130 further comprises a securing device for securing the printing plate P around the plate cylinder 131, an ink supply device, a water supply device, and a printing paper feeding device.
The printing plate P is positioned on the plate cylinder 131 by bringing the punched holes Pi and Pj into engagement with the first and second printing registration pins 136 and 137, respectively, and is then secured thereon by the securing device not shown. Thereafter, the ink supply device and the water supply device transfer the image IM (C) from the printing plate P to a printing sheet fed from the printing paper feeding device.
The printing plate P is produced by cutting a plate of metal, e.g. aluminum, to length and forming a layer for image formation on the surface thereof. Generally, the printing plate P is produced so that the edge Pe is straight. In some cases, however, the edge Pe has an undulation or swell Pw, as shown in FIG. 23A, due to low manufacturing accuracy or external factors after the manufacture. Thus, the printing plate P having the edge Pe which is not straight causes misregistration of the multi-color image formed on the printing sheet, which will be described in detail.
A problem arising from the operation using the image recorder 110, the punching apparatus 120 and the printing apparatus 130 discussed above will be described with reference to FIGS. 23A, 23B and 23C. It is assumed that the undulation or swell Pw is formed at a portion of the edge Pe of the printing plate P which is to come into contact with the second punching registration pin 127.
The printing plate P is positioned on the recording drum 111 of the image recorder 110 and is then secured thereon in the above-mentioned procedure. In this process, since the undulation or swell Pw on the edge Pe of the printing plate P does not contact the first and second pins 116 and 117, the printing plate P is properly positioned, and the proper image IM is recorded on the printing plate P. It is assumed that the recorded image IM is an image IM (M) corresponding to the magenta (M) color in this case.
The image is properly recorded on the printing plate P in this manner. However, the printing plate P is secured in an inclined orientation on the punch table 121 of the punching apparatus 120 since the printing plate P is positioned with the undulation or swell Pw in contact with the second punching registration pin 127. This produces the punched holes Pi and Pj displaced from their intended locations.
In the printing apparatus 130, the printing plate P is positioned by bringing the displaced holes Pi and Pj into engagement with the first and second printing registration pins 136 and 137. Thus, the image IM (M) recorded on the printing plate P assumes an angular orientation different from its intended angular orientation when the printing plate P is mounted on the plate cylinder 131. This results in misregistration when the image IM (M) of the magenta color is printed over the image IM (C) of the cyan color on the printing sheet.
Although the punched holes for use in the printing apparatus are produced in the punching apparatus 120 separate from the image recorder 110 in the above-mentioned instance, the above-mentioned problem arises also when the punching apparatus 120 is provided in the image recorder. Additionally, there is a likelihood that similar misregistration occurs when an image is formed with reference to the plate edge and the plate is punched by means of another punching apparatus.